Sounds like runway restrictions. Lands at MAN with empty tanks, so needs less runway. Takes off from MAN with full tanks, so needs to have less weight in PAX. Only problem is, MAN has a 3048m runway, and a300s all need about 2700m of runway.
Would put it in the bug report forum, see what sami says. Possibly the system thinks your plane needs much more runway than the plane info says, or the system thinks MAN has a shorter runway than the airport info says.